In a previous entry, I introduced the Rare Earth hypothesis of Peter Ward and Donald Brownlee, which states that Earth-like planets on which complex life could have evolved are very rare in the universe. One reason was that Earth revolves around a stable star, the Sun.
Ward and Brownlee also point out that the Earth resides in a very lucky neighborhood of the Solar System. The two sources of luck are Jupiter and the Moon. First, consider Jupiter.
When the Solar System first formed, it was a disc of small asteroids. Many of these asteroids ran into each other and were crushed into planets by their own gravity. These planets continued to mop up asteroids until about 3.9 billion years ago. After that time, few asteroids remained that could crash into the planets. Most of the craters on the Moon (which, as large as some planets, also helped to clear away asteroids) are older than 3.9 billion years. The Moon, which has no wind or weather, has preserved an intact sample of the asteroid impacts that imperiled the early Solar System.
Another important component of the Solar System is comets. There are billions of these dirty balls of ice that orbit the sun just beyond the outer edge of the Solar System. Most of them remain at the edge of the Solar System, but some of them have very elliptical orbits, which bring them close to the sun. They whip around the Sun like a slingshot, and fly back out into the outer edges of the Solar System. While comets are near the sun, solar radiation vaporizes some of the water, creating the comet’s “tail” that everyone recognizes. Before 3.9 billion years ago, there were also a lot of comets, but they are now, like asteroids, comparatively rare.
The principal reason that asteroids and comets now only rarely fall from the sky is the planet Jupiter. Jupiter is so massive, and has such a powerful gravitational field, that it has sucked up most of the asteroids in the inner solar system, except for those in the asteroid belt, whose orbits have been stabilized by that same Jovian gravitation. Any asteroid or comet that happens to come within several million miles of Jupiter is drawn inevitably into its gaseous embrace. This is exactly what happened to the comet Shoemaker-Levy 9 in 1994. After whipping around the Sun and heading back into the outer reaches of the solar system, this comet slipped too close to Jupiter, whose gravity fractured it into pieces. Each piece created a huge flare of radiation as it fell into Jupiter’s dense atmosphere, and each of the black spots that remained visible for a few weeks was similar in size to the Earth. Therefore Jupiter continues to clear away asteroids and comets from the Solar System. Without Jupiter, asteroids and comets might be hitting Earth so frequently that life would not have a chance to exist for very long.
And then there is Earth’s closest neighbor, the Moon. Most planets have moons, but Earth is the only planet in the Solar System with a moon so large in relation to it. Mars has two tiny moons, Deimos and Phobos, named after the two horses of the war god’s chariot. Jupiter and Saturn have moons larger than ours, but tiny in relation to the planetary masses. Our Moon is large enough and just far enough away to profoundly influence our planet without severely disrupting it. Everyone knows that the tug of the Moon causes the tides. Were it not for tides, there would be no intertidal zone, the only home of thousands of species of organisms. But tides may be of relatively little importance to the planet as a whole, even though they are important to barnacles. The major effect of the Moon on Earth, crucial to the survival of life as a whole, is to stabilize its movement.
As planets revolve around their suns, they rotate on their axes. These rotational axes wobble, pointing in different directions at different times. Any planet with a large amount of wobbling would have unstable climatic zones, since sometimes the equatorial zone and sometimes the polar zones would directly face the sun. The part of a planet directly facing its sun will receive the most intense radiation and be warmest. How could tropical, temperate, and polar plants and animals evolve, if the climates of those zones are extremely variable? This appears to have happened with Earth’s less fortunate little brother, Mars. Earth, however, has not tilted more than about 20 degrees from the plane of its revolution. Even the little bit of wobbling that the Earth does experience has been enough to cause about twenty Ice Ages during the last two million years of Earth history. We have the Moon to thank for the relative stability of Earth’s movements.
Earth is mighty lucky to have neighbors like Jupiter and the Moon. Otherwise, complex life might never have evolved here.
I adapted this essay from part of chapter 1 of my forthcoming book, Life of Earth: Portrait of a Beautiful, Middle-aged, Stressed-out World, to be released soon by Prometheus Books.
Also do not forget the new YouTube channel that I announced in the previous post (see below).